The Doubly Silent Ones with Their Outside Shadows

What I saw about me none else saw; and I grew doubly silent and aloof lest I be thought mad. Dogs had a fear of me, for they felt the outside shadow which never left my side...

The Book,

by H. P. Lovecraft

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Doubly-Silent One
No. Enc.: 1
Alignment: Neutral
Movement: 
Armor Class: 7 (As Leather)
Hit Dice: 2 (Advance as Dual-Class Spell-caster/Thieves)
Attacks: 1 (Weapon or Spell)
Damage: 1d4 or by Weapon or Spell
Save: T2
Morale: 5

Special: -4 penalty to Reaction Rolls with all dogs and guard animals. Apply both INT and DEX bonuses to Thief abilities. Make No noise when they move. While Skulking (before any attack/interaction) they can seamlessly blend into shadows. Once they attack or interact in any capacity, they must actively make an effort to Hide in Shadows (as a Thief), but can only do so if they no longer cast spells or seek to attack. Cannot use verbal means of casting spells - their gestural approach requires twice as long to cast than normal means. Darkness-related spells heal them at a rate of 1hp regained per level of caster. Light-based spells prevent them from healing during time of exposure. 

Furtive and skulking, those who become Doubly-Silent eschew normal modes of speech as too vulgar and inexact for their arcane needs. Unable to use verbal means of spell-casting, they rely instead upon a more deliberate and gestural approach to their spells, which takes twice as long to cast any given spell than usual, which is why they often prefer to focus on rituals and rites over the more immediate spells employed by adventurers.

They are notorious for keeping elaborately detailed journals, notebooks and spellbooks filled with all manner of esoteric trivia and cultic gibberish. These books are difficult to read, at best, and often require use of Comprehend Languages or similar spells to wade through their dense, purple prose and idiosyncratic nonsense. Doing so runs a risk of incurring madness or being cursed to require twice as long to read anything ever again, including scrolls or one's own grimoire. Any spells gained from these sources require twice as long to transcribe and can only be cast silently, meaning they take twice as long to cast as well. Each such spell transferred into an adventurer's spellbooks (or to scrolls) incurs a cumulative 30% chance of drawing the attention of an Outside Shadow that will begin to follow that character, patiently waiting for an opportunity to make them into a fresh host.

Outside Shadow
No. Enc.: 1
Alignment: Neutral (Evil)
Movement: 60' (20') Can climb like a spider.
Armor Class: 7 (as studded leather)
Hit Dice: 2 (Can 'heal' by draining the hit points of their Caller or Master, so long as the Caller/Master is sleeping)
Attacks: 1
Damage: 1d4, or Special
Save: T1
Morale: 10 (Gains +1 for each hit point sacrificed to them by their Caller or Master)

Special: Will do nearly anything to avoid bright light. Can adhere to nearly any material surface that is not Blessed. These unholy things delight in reaching into the mind of their chosen victim and sowing confusion and madness. They can also forgo gaining more hit points and instead develop spell slots and copy/memorize the spells their host, Caller or Master has prepared or known, with a cumulative 20% chance that the spell will be lost to both for each level of the spell concerned.

Take 1d4 from Holy Water or if touched by a Holy Symbol. If destroyed, they can attempt to 'resurface' through the foolish one who Called them or attempted to make them a servant.

Clingy, occult parasites that that freely climb and gambol about on any surface, so long as it is dimly lit or in the dark. Slow, weak and frail things, at least at first, these are not the usual Shadows one might have encountered before. These creatures bond with someone foolish enough to serve as their host, or who acts as their Caller via an occult ritual best not discussed in the open, or they will feign loyalty to a supposed Master for so long as this spell-caster allows the Outside Shadow to slowly feed upon their vitality while the Master sleeps (starting with 1-2 hit points a night). Over time the Shadow grows more greedy and begins to drain 1 more point each night, allowing them to gain a bonus of one permanent hit point for every 10 hit points drained from a willing host, Caller, or Master.

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Source: Right out of the quote above from HPL's The Book.